Our studies in the acute alveolar injury in the dog, caused by a single subcutaneous injection of n-nitroso-N-methylurethane suggest that the clinical and physiologic abnormalities are due principally to necrosis of alveolar epithelial cells and that recovery is due principally to regeneration of these cells. We are making a collaborative study of the injury from induction through recovery using physiologic, electron microscopic and biochemical techniques to examine the hypothesis that loss of mechanical integrity and surface activity attending epithelial necrosis are the common denominators in the pathodynamics, and to study the altered lung mechanics and the cellular and metabolic mechanism of recovery and the influence of adrenal corticosteroids on these mechanisms.